Mobile communication system, core network node selection method, and base station and mobile station used therefor

ABSTRACT

A mobile communication system in which a plurality of base stations and a plurality of core network nodes are distributed in a plurality of areas, wherein a source base station or a mobile station in a handover procedure of the mobile station transmits information for selecting a core network node to a target base station.

INCORPORATION BY REFERENCE

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2006-225967 filed on Aug. 23, 2006, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mobile communication system, a corenetwork node selection method, and a base station and a mobile stationused therefor, and more particularly, to a method for selecting a corenetwork node at the time of a handover associated with the movement of amobile station.

2. Description of Related Art

A mobile communication system based on Long Term Evolution (LTE) of 3GPPaims at such architecture as shown in FIG. 8. This architecture isintended to build a system that can realize data transfer of higherthroughput for example, than existing systems by reducing transmissiondelay in a control plane and a user plane.

In this architecture, when cells of base stations (eNodes B21 to 23) aresmall, e.g., in a metropolitan area, it is possible that movement of amobile station (User Equipment or UE) between base stationssignificantly increases handover signals transmitted from a base stationto a core network node (CN Node 11 or 12, hereinafter abbreviated as CNnode). Consequently, excess signal load would be placed on a CoreNetwork (CN). To reduce load of handover signals on the CN node, directexchange of handover signals between a source base station (Source eNodeB) and a target base station (Target eNode B) is under consideration.

Also, as shown in the sequence diagram of FIG. 9 that shows operationsduring a handover, when a mobile station moves from a cell covered by asource base station to a cell covered by a target base station and thesource base station carries out a handover (S1), if a handover issuccessful by using a handover signal (“Handover Request” message) S2which the target base station receives from the source base station,that is, if the target base station establishes a communicationconnection with the mobile station (S3 to S7), the handover procedurecan be completed just by the target base station transmitting one signalS8 called “Handover Complete” to a CN node.

The signal name “Handover Complete” is just an example: the signal isalso called “Path Switch”, “Binding Update” etch, all of which aresupposed to mean the same signal.

Upon receipt of the “Handover Complete” signal, the CN node switches thepath for the mobile station from the source base station to the targetbase station. In this way, the loads of signals and processing on CNnodes due to a handover can be reduced.

Describing this further in detail with reference to FIG. 9, S9 denotes aprocess for the CN node 11 having received the “Handover Complete”signal S8 to switch the path for the mobile station from the source basestation to the target base station After this process, the CN node 11transmits “Handover Complete Ack (Acknowledge)” signal S10 to the targetbase station. Upon receiving the signal S10, the target base stationtransmits “Release Resource” signal S11 to the source base station, andin response to the signal S11, the source base station releases a radioresource allocated for the UE. The release of resource is notified tothe CN node 11 with “Release Resource Indication” signal S12 to completethe handover process. In this manner, loads of signals and processing onCN nodes due to a handover can be reduced.

In a hierarchical structure of CN nodes and base stations, to preventservice unavailability in a wide area covered by one CN node in a corenetwork due to its system failure, for example, such a configuration hasbeen proposed in which CN nodes 11, 12 and eNode Bs 21 to 23, which arebase stations, are connected with one another in a mesh configuration sothat even if one of the CN nodes fails, the other CN node replaces it tocontinue to provide services, as shown in FIG. 8. This meshconfiguration is called “S1-Flex”, which is known from 3GPP TR 23.236V6. 3.0 (2006-03) (Non-Patent Document 1). The “S1” denotes S1 interfacewhich is an interface between CN nodes and base stations (eNode B), and“S1-Flex” used to be called “Iu-Flex” in mobile communication systemsprior to the LTE system.

In the LTE-based mobile communication system having the S1-Flexconfiguration, once a mobile station is connected to a CN node, it isbasically desirable not to change the CN node as long as thecommunication continues. This is because it is possible to reduceinterruption of communication due to a handover by minimizing handoversbetween CN nodes that are performed along with the movement of themobile station, and also because maintaining a communication path for along time period is desirable for a non-real-time service communicationsuch as data communication.

Existing systems have the soft handover function, which can preventdiscontinuity or interruption of audio even when the mobile stationmoves between base stations. However, the new LTE system mentioned aboveis designed to build architecture without soft handover because thesystem is complicated by maintaining the soft handover function.

The new LTE system also aims at the effect of reducing signal load on CNnodes by reducing handovers between CN nodes so that fewer signals areexchanged between CN nodes when a mobile station moves.

However, in the S1-Flex configuration shown in FIG. 8, it is sometimesimpractical not to change the CN node with which a mobile station shouldcommunicate when the mobile station moves. This is because, by way ofexample, if a mobile station as a user terminal starts communication inTokyo and continues to be connected to a CN node in Tokyo even after itsuser travels to Osaka by Shinkansen, the communication path gets longand transmission delay will significantly increase. In such a case, itis therefore required to change the CN node as the mobile station moves.Consequently, the function to change or not to change a CN node asappropriate when a mobile station moves is required for the S1-Flexconfiguration.

However, at present, there is no means or way for a target base stationto select a CN node at the time of a handover in the LTE system. In theexisting system, a CN node receives a handover request signal from asource RNC (radio network controller) and the CN node itself can judgewhether to change the CN node.

SUMMARY

An exemplary object of the invention is to provide a mobilecommunication system and a CN node selection method that enable a targetbase station to select an appropriate CN node when a mobile stationmoves, as well as a base station and a mobile station used for the same.

A base station according to an exemplary aspect of the invention is asource base station in a mobile communication system in which aplurality of base stations and a plurality of core network nodes aredistributed in a plurality of areas, the source base station including:

a wireless communication unit that performs wireless communication witha mobile station that performs handover from the source base station toa target base station in a handover procedure; and

a communication unit that transmits, to the target base station, a firstidentifier of a core network node to which the source base station isconnected and a second identifier of an area to which the source basestation belongs, in the handover procedure.

A base station according to an exemplary aspect of the invention is atarget base station in a mobile communication system in which aplurality of base stations and a plurality of core network nodes aredistributed in a plurality of areas, the target base station including:

a wireless communication unit that performs wireless communication witha mobile station that performs handover from a source base station tothe target base station in a handover procedure;

a communication unit that receives, from the source base station, afirst identifier of a core network node to which the source base stationis connected and a second identifier of an area to which the source basestation belongs, in the handover procedure.

A system according to an exemplary aspect of the invention is a mobilecommunication system in which a plurality of base stations and aplurality of core network nodes are distributed in a plurality of areas,the mobile communication system including;

a mobile station; a source base station; and a target base station,wherein

the mobile station that performs handover from the source base stationto the target base station in a handover procedure;

the source base station that transmits a first identifier of a corenetwork node to which the source base station is connected and a secondidentifier of an area to which the source base station belongs, in thehandover procedure; and

the target base station that receives the first identifier and thesecond identifier from the source base station.

A method according to an exemplary aspect of the invention is a methodfor a mobile communication system in which a plurality of base stationsand a plurality of core network nodes are distributed in a plurality ofareas, the method including:

in a source base station, performing wireless communication with amobile station that performs handover from the source base station to atarget base station in a handover procedure; and

in the source base station, transmitting, to the target base station, afirst identifier of a core network node to which the source base stationis connected and a second identifier of an area to which the source basestation belongs, in the handover procedure.

A mobile station according to an exemplary aspect of the invention is amobile station in a mobile communication system in which a plurality ofbase stations and a plurality of core network nodes are distributed in aplurality of areas, the mobile station including:

a handover processing unit that performs handover from a source basestation to a target base station in a handover procedure;

a wireless communication unit that performs wireless communication withthe source base station and the target base station; and

a communication unit that transmits, to the target base station, anidentifier of a core network node to which a source base station isconnected and an identifier of an area to which the source base stationbelongs, in the handover procedure.

A base station according to an exemplary aspect of the invention is atarget base station in a mobile communication system in which aplurality of base stations and a plurality of core network nodes aredistributed in a plurality of areas, the target base station including:

a wireless communication unit that performs wireless communication witha mobile station that performs handover from a source base station tothe target base station in a handover procedure;

the communication unit that receives, from a mobile station, a firstidentifier of a core network node to which the source base station isconnected and a second identifier of an area to which the source basestation belongs, in the handover procedure.

A system according to an exemplary aspect of the invention is a mobilecommunication system in which a plurality of base stations and aplurality of core network nodes are distributed in a plurality of areas,the mobile communication system including:

a source base station;

a target base station; and

a mobile base station that performs handover from the source basestation to the target base station in a handover procedure andtransmits, to the target base station, a first identifier of a corenetwork node to which a source base station is connected and a secondidentifier of an area to which the source base station belongs, in thehandover procedure.

A method according to an exemplary aspect of the invention is a methodfor a mobile communication system in which a plurality of base stationsand a plurality of core network nodes are distributed in a plurality ofareas, the method including:

in a mobile station, performing handover from a source base station to atarget base station in a handover procedure; and

in the mobile station, transmitting, to the target base station, a firstidentifier of a core network node to which the source base station isconnected and a second identifier of an area to which the source basestation belongs, in the handover procedure.

A system according to an exemplary aspect of the invention is a mobilecommunication system in which a plurality of base stations and aplurality of core network nodes are distributed in a plurality of areas,wherein

a target base station in a handover procedure of a mobile stationselects a core network node to which the target base station connectsbased on information for selecting a core network node transmitted froma source base station or a mobile station.

A system according to an exemplary aspect of the invention is a mobilecommunication system in which a plurality of base stations and aplurality of core network nodes are distributed in a plurality of areas,wherein

a source base station or a mobile station in a handover procedure of themobile station transmits information for selecting a core network nodeto a target base station.

A base station according to an exemplary aspect of the invention is asource base station in a mobile communication system including a targetbase station and a mobile station, the source base station including:

a wireless communication unit that performs wireless communication withthe mobile station that performs handover from the source base stationto the target base station in a handover procedure; and

a communication unit that transmits an identifier of a core network nodeto which the source base station is connected to the target base stationin the handover procedure.

A system according to an exemplary aspect of the invention is a mobilecommunication system including a source base station, a target basestation and a mobile station, wherein

the mobile station performs handover from the source base station to thetarget base station in a handover procedure,

the source base station performs wireless communication with the mobilestation and transmits an identifier of a core network node to which thesource base station is connected to the target base station in thehandover procedure, and

the target base station receives the identifier of the core network.

A method according to an exemplary aspect of the invention is a methodfor a communication system, the system including a source base station,a target base station and a mobile station, the method including:

performing, in the source base station, wireless communication with themobiles station that performs handover from the source base station tothe target base station in a handover procedure; and

transmitting, in the source base station, an identifier of a corenetwork node to which the source base station is connected, to thetarget base station in the handover procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram showing a configuration of pool areas towhich the exemplary embodiments of the invention are applied;

FIG. 2 is a functional block diagram of a base station according to afirst exemplary embodiment of the invention;

FIG. 3 is a functional block diagram of a mobile station according to afirst exemplary embodiment of the invention;

FIG. 4 is a sequence diagram showing operations in the first exemplaryembodiment of the invention;

FIG. 5 is a flowchart illustrating the algorithm for selecting a CN nodefor communication executed at a target base station in the firstexemplary embodiment of the invention;

FIG. 6 is a flowchart illustrating the algorithm for selecting a CN nodefor communication executed at a target base station in the secondexemplary embodiment of the invention;

FIG. 7 is a functional block diagram of a mobile station according to athird exemplary embodiment of the invention;

FIG. 8 illustrates the existing S1-Flex configuration among eNode Bs andCN nodes; and

FIG. 9 is a sequence diagram showing an example of a handover procedurein the configuration of FIG. 8.

EXEMPLARY EMBODIMENT

The present invention will be described below with reference todrawings. FIG. 1 is a system configuration diagram for illustrating theprinciple of the exemplary embodiments of the invention, showing anexample of a mobile communication system that uses the concept of “PoolArea” which is defined in the Non-patent Document 1. In FIG. 1,components corresponding to the ones shown in FIG. 8 are denoted withthe same reference numerals as in FIG. 8.

A CN is responsible for call control, location control, service controland the like, and sometimes collectively refers to an Access Gateway(ASGW) and/or an Access Anchor. CN nodes 11 to 13 are nodes constitutinga Core Network (CN). The CN nodes may be nodes having known functions,such as MSC (Mobile Switching Center), SGSN (Serving GPRS (GeneralPacket Radio Service)Support Node) and HLR (Home Location Register). A“CN” sometimes refers to a CN node itself.

A certain area covered by the CN nodes 11 to 13 and base stations 21 to24 is segmented into pool areas 41 and 42, each of which has assignedthereto a pool area ID for identifying the area. It is assumed that thepool areas 41 and 42 are defined in advance by an operator. Each of theCN nodes 11 to 13 and base stations 21 to 24 has a pool area ID of thepool area to which it belongs. To each of the CN nodes, a CN node ID isassigned, and to each of the base stations, a base station ID isassigned.

In FIG. 1, in the pool area 41, the CN nodes 11 and 12, and the basestations 21 to 23 are located, and in the pool area 42, the CN node 13and the base station 24 are located It is assumed that a mobile stationnot shown is now in communication with the base station 21 and the basestation 21 is being connected to and communicating with the CN node 11.In this situation, when the mobile station moves to a cell of the basestation 22, the base station 22 remains to be connected to the CN node11 and does not change the CN node for connection because the basestation 22 is still able to connect to the CN node 11.

It is assumed that the mobile station moves from the base station 22 tothe base station 23 while still in communication. At this time, sincethe distance between the base station 23 and the CN node 11 is stillshort enough not to affect delay in communication, the base station 23remains to be connected to the CN node 11 and does not change the CNnode for connection.

It is assumed that the mobile station further moves from the basestation 23 to the base station 24 while in communication Then, thedistance between the target base station 24 and the CN node 11 in thepool area 41 becomes long enough to affect delay in communication, sothat the target base station 24 determines whether to change the CN nodein accordance with the type of communication performed by the mobilestation. If the communication is of a type that is not sensitive todelay, e.g., a non-real-time service (NRT) type, the base station 24continues to communicate with the CN node 11.

However, if the communication performed by the mobile station is of areal-time (RT) service type that is sensitive to delay, the base station24 selects the CN node 13 belonging to the pool area 42, to which thebase station 24 belongs, as a CN node for communication (connection),and continues communication. Here, the non-real-time service may be datacommunication, for example, and the real-time service may be audiocommunication and/or streaming, for instance.

Based on the principle described above, a first exemplary embodiment ofthe invention can be provided as follows. FIG. 2 is a functional blockdiagram of a base station according to the first exemplary embodiment ofthe invention. The base station according to the first exemplaryembodiment of the invention includes a wireless communication unit 31, acommunication unit 32, a handover processing unit 33, a CN nodeselection unit 34, a notification information generating unit 35, acontrol unit (CPU) 36, and a memory 37.

The wireless communication unit 31 performs communication with mobilestations. The communication unit 32 performs communication with CN nodesand/or other base stations. The handover processing unit 33 performshandover processing. The CN node selection unit 34 selects a CN node fora mobile station to communicate with (or connect to) when the basestation becomes a target base station at the time of a handover of themobile station. The notification information generating unit 35generates notification information for a target base station when thebase station becomes a source base station at the time of a handover.The control unit (CPU) 36 controls the units 31 to 35. The memory 37functions as work memory for the control unit 36 and also as ROM inwhich the control operations of the control unit 36 are prestored as acontrol procedure program.

FIG. 3 is a functional block diagram of a mobile station according tothe first exemplary embodiment of the invention. The mobile station ofthe first exemplary embodiment of the invention includes a wirelesscommunication unit 51, a handover processing unit 52, a control unit(CPU) 53, and a memory 54. The wireless communication unit 51 performscommunication with a base station. The handover processing unit 52performs handover processing. The control unit (CPU) 53 controls theunits 51 and 52. The memory 54 functions as work memory for the controlunit 53 and also as ROM in which the control operations of the controlunit are prestored as a control procedure program.

FIG. 4 is a sequence diagram showing operations in the first exemplaryembodiment of the invention, wherein signals and processingcorresponding to the ones shown in FIG. 9 are denoted with the samereference numerals as in FIG. 9. When the mobile station moves from acell of a source base station to a cell of a target base station and thesource base station performs a handover (S1), the source base stationtransmits “Handover Request” message S2 as a handover request signal tothe target base station. The message S2 contains information forselecting a CN node, such as the type of communication performed by themobile station, the ID of a CN node with which the source base stationhas been communicating, and the ID of a pool area (PA) to which thesource base station belongs. The information for selecting a CN node isgenerated by the notification information generating unit 35 shown inFIG. 2.

After the handover succeeds and the target base station establishes acommunication connection with the mobile station (S3 to S7), the targetbase station selects a CN node to which it should be connected inaccordance with a CN node selection algorithm at the CN node selectionunit 34 (see FIG. 2) (S21). The flowchart shown in FIG. 5 illustrates anexample of this CN node selection algorithm. Referring to FIG. 5, thetype of communication performed by the mobile station which will undergoa handover is first determined (step S31). Specifically, it isdetermined whether the communication type is a real-time service or anon-real-time service. This determination is made by using communicationtype information contained in the “Handover Request” signal S2.

If the communication type is a non-real-time service, the target basestation selects the CN node with which the source base station has beencommunicating (been connected to), that is to say, does not change theCN node (step S33). In this case, subsequent processing will be the sameas processing S8 to S12 of FIG. 9. On the other hand, if thecommunication type is a real-time service such as audio communication,it is determined whether the pool area ID of the source base station isthe same as that of the target base station (step S32).

This determination is made with information on the ID of a pool area towhich the source base station belongs that is contained in the “HandoverRequest” signal S2. If the pool area ID of the source base station isthe same as that of the target base station (Yes at step S32),processing at step S33 is performed, that is, the CN node is notchanged.

On the other hand, if the pool area ID of the source base station isdifferent from that of the target base station, processing at step S34is performed. In this processing, a CN node that is closer to the targetbase station than the CN node with which the source base station hasbeen communicating (or connected to) (generally, a CN node within a poolarea to which the target base station belongs) is selected as the nodefor connection. This can prevent information delay that results from along communication path. Here, the target base station can also select aCN node that is closest to it. When selecting a CN node, the target basestation may be configured to select one with reference to a tableshowing the positional relationship between the target base station andCN nodes which is prepared in each base station.

Referring back to FIG. 4, after step S34 of FIG. 5, the target basestation transmits “Handover Complete” message S22 to the selected CNnode. Assuming that the target base station is the base station 24 inthe system of FIG. 1, the selected CN node is the CN node 13 within thesame pool area 42. Therefore, the sequence of FIG. 4 shows that thesignal S22 is transmitted to the CN node 13.

After the CN node 13 receives the “Handover Complete” message S22 fromthe target base station, the CN node 13 asks the source CN node (CN node11 in this example) for information on the mobile station, that is, “UEContext” (“Context Request” signal S23), because the CN node 13 does nothave control information for the mobile station. The target CN node 13can get the destination of the “Context Request” signal S23 from the IDof the source CN node 11 contained in the “Handover Complete” messageS22 received from the target base station. Then, the CN node 13 receivesthe “UE Context” information through “Context Response” signal S24 fromthe CN node 11, sets the target base station and a path, and continuescommunication (S25, S11, and S12).

The selection of a CN node at the target base station shown in FIG. 5can be represented as the rules below:

-   If QoS=NRT (if Quality of Service (QoS) is a non-real-time service),-   then, target eNode B chooses same CN Node.-   If QoS=RT and PA=own PA (if the QoS is a real-time service and the    pool area ID is the same as its own pool area ID),-   then, target eNode B chooses same CN Node,-   else,-   if QoS=RT and PA≠own PA (if the QoS is a real-time service and the    pool area ID is not the same as its own pool area ID),-   then, target eNode B chooses target CN Node.

By selecting a CN node based on a communication type and a pool area,the first exemplary embodiment provides the effects of reducing thefrequency of CN node switching and lessening the influence ofinformation delay resulting from a long communication path on thequality of a service which is sensitive to delay, such as a real-timeservice.

The first exemplary embodiment described above focuses attention todelay of information resulting from a long communication path anddetermines whether the communication type is a real-time service (RT) ora non-real-time service (NRT), and changes the CN node if thecommunication type is a real-time service which is sensitive to delayand if the pool ID is different. Next, a second exemplary embodimentwill be described below as illustration of a case that focuses ondiscontinuity (or interruption) of information due to CN node switchingand controls CN node switching by determining whether the communicationtype is an audio communication service which is sensitive todiscontinuity of information or a data communication service whichtolerates discontinuity of information.

As the system configuration, and the configurations of a base stationand a mobile station of the second exemplary embodiment are the same asthose of the first exemplary embodiment, description of them is omitted.The operation sequence diagram of this exemplary embodiment is the sameas the operation sequence of FIG. 4 except for the algorithm for CN nodeselection processing S21 at the target base station in FIG. 4.

FIG. 6 is a flowchart showing the CN node selection algorithm in thisexemplary embodiment, wherein steps corresponding to the ones FIG. 5 aredenoted with the same reference numerals as in FIG. 5. In FIG. 6, thetype of communication performed by the mobile station which is toundergo a handover is determined (step S31). Specifically, at step S31,it is determined whether the communication type is an audiocommunication service which is sensitive to discontinuity of informationor a data communication service which tolerates discontinuity ofinformation.

If the communication type is an audio communication service, the targetbase station selects the CN node with which the source base station hasbeen communicating, that is, does not change the CN node (step S33).This can prevent discontinuity of information due to switching of a CNnode. On the other hand, if the communication type is a datacommunication service, it is determined whether the pool area ID of thesource base station is the same as that of the target base station (stepS32). If the pool area ID of the source base station is the same as thatof the target base station, processing at step S33 is performed, i.e.,the CN node is not changed. If the pool area ID is different, processingat step S34 is performed, where a CN node which is closer to the targetbase station than the CN node with which the source base station hasbeen communicating (or connected to) (generally, a CN node within thepool area to which the target base station belongs) is selected as anode for connection.

As in the first exemplary embodiment, the target base station may beconfigured to reference a table showing the positional relationshipbetween it and CN nodes which is prepared in each base station whenselecting a CN node closest to it (target base station) and/or a CNnode.

In this case, a CN node is selected at the target base station inaccordance with the rules below:

-   If QoS=Audio (if the QoS is an audio service),-   then, target eNode B chooses same CN Node.-   If QoS=Data and PA=own PA (if the QoS is data communication and the    pool area ID is the same as its own pool area ID),-   then, target eNode B chooses same CN Node,-   else,-   if QoS=DATA and PA≠own PA (if the QoS is data communication and the    pool area ID is not the same as its own pool area ID),-   then, target eNode B chooses target CN Node.

According to the second exemplary embodiment, by selecting a CN nodebased on a communication type and a pool area, there can be provided aneffect of lessening the influence of information discontinuity due toswitching of a CN node on the quality of communication that is sensitiveto discontinuity of information, such as audio communication, whilepresupposing switching of a CN node according to movement of a mobilestation.

Which of the CN node selection algorithm of the first exemplaryembodiment (FIG. 5) and that of the second exemplary embodiment (FIG. 6)to adopt can depend on the system in question. For example, for a systemwhich puts emphasis on the reduction of influence of information delayresulting from a long communication path on communication quality, theCN node selection algorithm of the first exemplary embodiment may beadopted For a system which puts emphasis on the reduction of influenceof information discontinuity due to CN node switching on communicationquality, the CN node selection algorithm of the second exemplaryembodiment may be adopted.

As has been described, according to the first or second exemplaryembodiment, when the “Handover Request” message contains CN node IDinformation and pool area ID information, the target base station canuse those pieces of information with communication type information todecide an optimum CN node.

The communication types (QoS) in the first and second exemplaryembodiments described above are merely examples and are not limited tothe ones described: the communication type (QoS) information may also bea guaranteed bit rate, maximum bit rate, delay, error rate, trafficclass (e.g., audio, streaming, background, interactive), for example. ACN node selection algorithm appropriately defined to such communicationtypes can be applied to the first and second exemplary embodiments.

It is also possible to define an information set which consists ofelements of information on a plurality of communication types (Qos)(e.g., guaranteed bit rate, maximum bit rate, delay, error rate, andtraffic class). It is also possible to define a label for indicating acombination of values of elements in such an information set (e.g.,combination of guaranteed bit rate=A(bps), maximum bit rate=B(bps) andtraffic class=audio). In this case, information on communication type(QoS) may be transmitted as a label (e.g., one-byte information) from aCN node to an eNode B.

The label can also be used as the communication type contained in the“Handover Request” message in the sequence of FIG. 4 which istransmitted from a source base station to a target base station. In thiscase, the target base station can select an appropriate piece ofcommunication type (QoS) information (element) (e.g., trafficclass=audio) from the elements of an information set about communicationtype (QoS) received as a label, and use the selected communication typeinformation in the CN node selection algorithm.

In such a way, even in a system that has many pieces of communicationtype (QoS) information, there is provided an effect of allowing a targetbase station to select an appropriate CN node while suppressing theamount or duration of communication between the source base station andthe target base station.

In the exemplary embodiments described above, the target base stationreceives the communication type of the mobile station, the ID of a CNnode with which the source base station is communicating, and a poolarea ID to which the source base station belongs, which are informationnecessary for CN node selection, in the “Handover Request” message S2(see FIG. 4) from the source base station. On the other hand, in a thirdexemplary embodiment of the invention, the target base station receivessuch information (i.e., information for CN node selection) through the“Handover Complete” message S7 shown in FIG. 4 from a mobile station.

The system configuration and mobile station configuration of the thirdexemplary embodiment are the same as those shown in FIGS. 1 and 2. FIG.7 shows the configuration of a mobile station, wherein unitscorresponding to the ones shown in FIG. 3 are denoted with the samereference numerals as in FIG. 3. As shown in FIG. 7, the mobile stationof the third exemplary embodiment has a notification information storingunit 55 for storing notification information for a target base stationin addition to the configuration of FIG. 3. The notification informationstoring unit 55 stores the communication type, the CN node ID with whichthe source base station is communicating, and the pool area ID of thesource base station. The mobile station includes the information storedin the notification information storing unit 55 in the “HandoverComplete” message S7 for transmission to the target base station. Thetarget base station receives the information and decides a CN node towhich it should connect in accordance with the CN node selectionalgorithm shown in FIGS. 5 or 6.

In the third exemplary embodiment, the mobile station transmitsinformation for CN node selection (communication type, and the ID andthe pool area ID of the CN node with which the source base station iscommunicating) to the target base station in the “Handover Complete”message S7. However, the message used by the mobile station fortransmitting the information for CN node selection to the target basestation is not limited to the “Handover Complete” message. In a systemwhere a mobile station, not the source base station, requests ahandover, for example, the mobile station can also use the “HandoverRequest”, which is a handover request signal, to transmit theinformation for CN node selection to the target base station.

In the exemplary embodiments described above, the target base stationmakes determination based on the pool area ID of the source base station(step S32) after determination based on communication type (step S31).However, the target base station can also make determination based oncommunication type (step S31) after determination based on the pool areaID of the source base station (step S32) In other words, step S31 andstep S32 can also be interchanged in FIGS. 5 and 6.

In the exemplary embodiments described above, the target base stationselects or decides a CN node with which it will communicate based on thecommunication type, the ID of the CN node with which the source basestation is communicating, and the ID of the pool area to which thesource base station belongs. However, the target base station can alsodecide a node for communication based on the CN node with which thesource base station is communicating and the ID of the pool area towhich the source base station belongs. In this case, the target basestation determines whether the ID of the pool area to which the sourcebase station belongs is the same as its pool area ID without consideringthe communication type. In other words, step S31 is removed from the CNselection algorithms shown in FIGS. 5 and 6.

This has the effect of allowing the target base station to select a CNnode while suppressing the amount of processing performed by the targetbase station. In addition, the source base station and/or the mobilestation may also be configured not to send a communication type to thetarget base station. This provides the effect of allowing the targetbase station to select a CN node while suppressing the amount orduration of communication between the source base station or the mobilestation and the target base station.

In the exemplary embodiments described above, the target base stationdecides appropriately a CN node for communication by using thecommunication type, the ID of the CN node with which the source basestation is communicating, and the ID of the pool area to which thesource base station belongs. However, it is also possible to arrange thesource base station and/or the mobile station to send only the CN nodeID to the target base station. In this case, the target base station canalso receive the CN node ID from the source base station and/or themobile station and connect to a CN node which has the CN node IDreceived.

This provides the effect of allowing the target base station to select aCN node while suppressing the amount or duration of communicationbetween the source base station or the mobile station and the targetbase station. It also has the effect of enabling the target base stationto select a CN node while suppressing the amount of processing to beperformed by the target base station.

Further, the source base station and/or the mobile station may alsodecide a CN node to which the target base station should connect takinginto consideration the communication type and/or the pool area ID of CNnodes in advance and send the ID of the CN node decided to the targetbase station. This provides the effect of enabling the source basestation and/or the mobile station to select a CN node to which thetarget base station should connect.

In addition, when executing the CN node selection algorithm shown inFIGS. 5 or 6, the target base station can select a CN node also inconsideration of information indicating load on CN nodes. In this case,if the load on a CN node decided or selected for connection is large(i.e., the amount of load is large), the target base station may alsoselect another CN node in its vicinity which is under less load. Thisprovides the effect of allowing the target base station to select anappropriate CN node also in consideration of load distribution among CNnodes in addition to information such as the pool area ID.

It will be apparent that the operation procedure of the base stationand/or the mobile station in the exemplary embodiments can be stored asprograms in a recording medium such as a ROM, which are read andexecuted by a computer (CPU).

Further, a fourth exemplary embodiment of the invention is a mobilecommunication system in which a plurality of base stations and aplurality of core network nodes are distributed in a plurality of areas,wherein a target base station at the time of a handover of a mobilestation selects a core network node to which the target base stationshould connect based on information for selecting a core network nodetransmitted from a source base station or the mobile station.

A fifth exemplary embodiment of the invention is a mobile communicationsystem in which a plurality of base stations and a plurality of corenetwork nodes are distributed in a plurality of areas, wherein a sourcebase station or a mobile station at the time of a handover of the mobilestation transmits information for selecting a core network node to atarget base station.

A sixth exemplary embodiment of the invention is a mobile communicationsystem in which a plurality of base stations and a plurality of corenetwork nodes are distributed in a plurality of areas, the mobilecommunication system comprising:

in a source base station at the time of a handover of a mobile station,

a message generating unit for generating a handover request messagewhich includes an identifier of a core network node to which the sourcebase station is connected and an identifier of an area to which thesource base station belongs,

and a transmitting unit for transmitting the message to a target basestation.

A seventh exemplary embodiment of the invention is a mobilecommunication system in which a plurality of base stations and aplurality of core network nodes are distributed in a plurality of areas,the mobile communication system comprising:

in a mobile station,

a message generating unit for generating a handover request messagewhich includes an identifier of a core network node to which a sourcebase station is connected and an identifier of an area to which thesource base station belongs,

and a transmitting unit for transmitting the message to a target basestation at the time of a handover.

A eighth exemplary embodiment of the invention is, in a mobilecommunication system in which a plurality of base stations and aplurality of core network nodes are distributed in a plurality of areas,a method for selecting a core network node to which a target basestation should connect at the time of a handover of a mobile station,comprising:

in a source base station,

generating a handover request message which includes an identifier of acore network node to which the source base station is connected and anidentifier of an area to which the source base station belongs,

and transmitting the message to a target base station.

A ninth exemplary embodiment of the invention is, in a mobilecommunication system in which a plurality of base stations and aplurality of core network nodes are distributed in a plurality of areas,a method for selecting a core network node to which a target basestation should connect at the time of a handover of a mobile station,comprising:

in the mobile station, including an identifier of a core network node towhich a source base station is connected and an identifier of an area towhich the source base station belongs into a message to a target basestation at the time of a handover, and transmitting the message.

A tenth exemplary embodiment of the invention is a source base stationat the time of a handover of a mobile station in a mobile communicationsystem in which a plurality of base stations and a plurality of corenetwork nodes are distributed in a plurality of areas, the source basestation comprising:

a message generating unit for generating a handover request messagewhich includes an identifier of a core network node to which the sourcebase station is connected and an identifier of an area to which thesource base station belongs,

and a transmitting unit for transmitting the message to a target basestation.

A eleventh exemplary embodiment of the invention is a target basestation at the time of a handover of a mobile station in a mobilecommunication system in which a plurality of base stations and aplurality of core network nodes are distributed in a plurality of areas,the target base station comprising:

a selection unit for selecting a core network node to which the targetbase station should connect based on an identifier of a core networknode to which a source base station is connected and an identifier of anarea to which the source base station belongs, which are contained in ahandover request message from the source base station.

A twelfth exemplary embodiment of the invention is a target base stationat the time of a handover of a mobile station in a mobile communicationsystem in which a plurality of base stations and a plurality of corenetwork nodes are distributed in a plurality of areas, the target basestation comprising:

a selection unit for selecting a core network node to which the targetbase station should connect based on an identifier of a core networknode to which a source base station is connected and an identifier of anarea to which the source base station belongs, which are contained in amessage at the time of the handover from the mobile station.

A thirteenth exemplary embodiment of the invention is a mobile stationin a mobile communication system in which a plurality of base stationsand a plurality of core network nodes are distributed in a plurality ofareas, the mobile station comprising:

a message generating unit for generating a message which includes anidentifier of a core network node to which a source base station isconnected and an identifier of an area to which the source base stationbelongs,

and a transmitting unit for transmitting the message to a target basestation at the time of a handover.

An exemplary advantage according to the invention is that a target basestation itself can appropriately select a CN node with which it shouldcommunicate by receiving information necessary for selecting a CN nodefrom a source base station or a mobile station at the time of thehandover of the mobile station.

While the invention has been particularly shown and described withreference to exemplary embodiments thereof, the invention is not limitedto these embodiments. It will be understood by those of ordinary skillin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present invention asdefined by the claims.

1. A source base station in a mobile communication system in which aplurality of base stations and a plurality of core network nodes aredistributed in a plurality of areas, the source base station comprising:a wireless communication unit that performs wireless communication witha mobile station that performs handover from the source base station toa target base station in a handover procedure; and a communication unitthat transmits, to the target base station, a first identifier of a corenetwork node to which the source base station is connected and a secondidentifier of an area to which the source base station belongs, in thehandover procedure.
 2. The source base station according to claim 1,wherein the first identifier and the second identifier are included in ahandover request message, and the communication unit transmits thehandover request to the target base station.
 3. The source base stationaccording to claim 1, wherein the communication unit transmitscommunication type information to the target base station in thehandover procedure.
 4. The source base station according to claim 3,wherein the communication type information is a label indicating valuesof communication types.
 5. The source base station according to claim 3,wherein the communication type information includes at least one of aguaranteed bit rate, a maximum bit rate, a delay, an error rate and atraffic class.
 6. A target base station in a mobile communication systemin which a plurality of base stations and a plurality of core networknodes are distributed in a plurality of areas, the target base stationcomprising: a wireless communication unit that performs wirelesscommunication with a mobile station that performs handover from a sourcebase station to the target base station in a handover procedure; acommunication unit that receives, from the source base station, a firstidentifier of a core network node to which the source base station isconnected and a second identifier of an area to which the source basestation belongs, in the handover procedure.
 7. The target base stationaccording to claim 6, wherein the first identifier and the secondidentifier are included in a handover request message, and thecommunication unit receives the handover request from the source basestation.
 8. The target base station according to claim 6, furthercomprising a selection unit that selects a core network node to whichthe target base station connects based on the first identifier and thesecond identifier.
 9. The target base station according to claim 6,wherein the communication unit receives communication type informationfrom the source base station in the handover procedure.
 10. The targetbase station according to claim 9, wherein further comprising aselection unit that selects a core network node to which the target basestation connects based on the first identifier, the second identifierand the communication type information.
 11. The target base stationaccording to claim 9, wherein the communication type information is alabel indicating values of communication types.
 12. The target basestation according to claim 9, wherein the communication type informationincludes at least one of a guaranteed bit rate, a maximum bit rate, adelay, an error rate and a traffic class.
 13. A mobile communicationsystem in which a plurality of base stations and a plurality of corenetwork nodes are distributed in a plurality of areas, the mobilecommunication system comprising; a mobile station; a source basestation; and a target base station, wherein the mobile station thatperforms handover from the source base station to the target basestation in a handover procedure; the source base station that transmitsa first identifier of a core network node to which the source basestation is connected and a second identifier of an area to which thesource base station belongs, in the handover procedure; and the targetbase station that receives the first identifier and the secondidentifier from the source base station.
 14. The mobile communicationsystem according to claim 13, wherein the first identifier, and thesecond identifier are included in a handover request message, and thecommunication unit transmits the handover request to the target basestation.
 15. The mobile communication system according to claim 13,wherein the target base station selects a core network node to which thetarget base station connects based on the first identifier and thesecond identifier.
 16. The mobile communication system according toclaim 13, wherein the source base station transmits communication typeinformation to the target base station in the handover procedure. 17.The mobile communication system according to claim 16, wherein thetarget base station selects a core network node to which the target basestation connects based on the first identifier, the second identifierand the communication type information.
 18. A method for a mobilecommunication system in which a plurality of base stations and aplurality of core network nodes are distributed in a plurality of areas,the method comprising: in a source base station, performing wirelesscommunication with a mobile station that performs handover from thesource base station to a target base station in a handover procedure;and in the source base station, transmitting, to the target basestation, a first identifier of a core network node to which the sourcebase station is connected and a second identifier of an area to whichthe source base station belongs, in the handover procedure.
 19. Themethod according to claim 18, wherein the first identifier and thesecond identifier are included in a handover request message, and thesource base station transmits the handover request to the target basestation.
 20. The method according to claim 18, further comprising: inthe target base station, selecting a core network node to which thetarget base station connects based on the first identifier and thesecond identifier.
 21. The method according to claim 20, wherein thesource base station transmits communication type information to thetarget base station in the handover procedure.
 22. The method accordingto claim 21, further comprising in the target base station, selecting acore network node to which the target base station connects based on thefirst identifier, the second identifier and the communication typeinformation.
 23. A mobile station in a mobile communication system inwhich a plurality of base stations and a plurality of core network nodesare distributed in a plurality of areas, the mobile station comprising:a handover processing unit that performs handover from a source basestation to a target base station in a handover procedure; a wirelesscommunication unit that performs wireless communication with the sourcebase station and the target base station; and a communication unit thattransmits, to the target base station, an identifier of a core networknode to which a source base station is connected and an identifier of anarea to which the source base station belongs, in the handoverprocedure.
 24. The mobile station according to claim 23, wherein thefirst identifier and the second identifier are included in a handovercompletion message, and the communication unit transmits the handovercompletion message to the target base station.
 25. The mobile stationaccording to claim 23, wherein the communication unit transmitscommunication type information to the target base station in thehandover procedure.
 26. A target base station in a mobile communicationsystem in which a plurality of base stations and a plurality of corenetwork nodes are distributed in a plurality of areas, the target basestation comprising: a wireless communication unit that performs wirelesscommunication with a mobile station that performs handover from a sourcebase station to the target base station in a handover procedure; thecommunication unit that receives, from a mobile station, a firstidentifier of a core network node to which the source base station isconnected and a second identifier of an area to which the source basestation belongs, in the handover procedure.
 27. The target base stationaccording to claim 26, wherein the first identifier and the secondidentifier are included in a handover completion message, and thecommunication unit receives the handover completion message from themobile station.
 28. The target base station according to claim 26,further comprising a selection unit that selects a core network node towhich the target base station connects based on the first identifier andthe second identifier.
 29. The target base station according to claim26, wherein the communication unit receives communication typeinformation from the mobile station in the handover procedure.
 30. Thetarget base station according to claim 29, wherein further comprising aselection unit that selects a core network node to which the target basestation connects based on the first identifier, the second identifierand the communication type information.
 31. A mobile communicationsystem in which a plurality of base stations and a plurality of corenetwork nodes are distributed in a plurality of areas, the mobilecommunication system comprising: a source base station; a target basestation; and a mobile base station that performs handover from thesource base station to the target base station in a handover procedureand transmits, to the target base station, a first identifier of a corenetwork node to which a source base station is connected and a secondidentifier of an area to which the source base station belongs, in thehandover procedure.
 32. The mobile communication system according toclaim 31, wherein the first identifier and the second identifier areincluded in a handover completion message, and the mobile stationtransmits the handover completion message to the target base station.33. The mobile communication system according to claim 31, wherein thetarget base station selects a core network node to which the target basestation connects based on the first identifier and the secondidentifier.
 34. The mobile communication system according to claim 31,wherein the mobile station transmits s communication type information tothe target base station in the handover procedure.
 35. The mobilestation according to claim 34, wherein the target base station selects acore network node to which the target base station connects based on thefirst identifier, the second identifier and the communication typeinformation.
 36. A method for a mobile communication system in which aplurality of base stations and a plurality of core network nodes aredistributed in a plurality of areas, the method comprising: in a mobilestation, performing handover from a source base station to a target basestation in a handover procedure; and in the mobile station,transmitting, to the target base station, a first identifier of a corenetwork node to which the source base station is connected and a secondidentifier of an area to which the source base station belongs, in thehandover procedure.
 37. The method according to claim 36, wherein thefirst identifier and the second identifier are included in a handovercompletion message, and mobile station transmits the handover completionmessage to the target base station.
 38. The method according to claim36, further comprising: in the target base station, selecting a corenetwork node to which the target base station connects based on thefirst identifier and the second identifier.
 39. The method according toclaim 36, wherein the mobile station transmits communication typeinformation to the target base station in the handover procedure. 40.The method according to claim 39, further comprising in the target basestation, selecting a core network node to which the target base stationconnects based on the first identifier, the second identifier and thecommunication type information.
 41. A mobile communication system inwhich a plurality of base stations and a plurality of core network nodesare distributed in a plurality of areas, wherein a target base stationin a handover procedure of a mobile station selects a core network nodeto which the target base station connects based on information forselecting a core network node transmitted from a source base station ora mobile station.
 42. A mobile communication system in which a pluralityof base stations and a plurality of core network nodes are distributedin a plurality of areas, wherein a source base station or a mobilestation in a handover procedure of the mobile station transmitsinformation for selecting a core network node to a target base station.43. A source base station in a mobile communication system comprising atarget base station and a mobile station, the source base stationcomprising: a wireless communication unit that performs wirelesscommunication with the mobile station that performs handover from thesource base station to the target base station in a handover procedure;and a communication unit that transmits an identifier of a core networknode to which the source base station is connected to the target basestation in the handover procedure.
 44. The source base station accordingto claim 43, wherein the identifier of the core network node is includedin a handover request message, and the communication unit transmits thehandover request message to the target base station.
 45. The source basestation according to claim 43, wherein the communication unit transmitscommunication type information to the target base station.
 46. Thesource base station according to claim 45, wherein the communicationtype information is a label indicating values of communication types.47. The source base station according to claim 45, wherein thecommunication type information includes at least one of a guaranteed bitrate, a maximum bit rate, a delay, an error rate and a traffic class.48. A mobile communication system comprising a source base station, atarget base station and a mobile station, wherein the mobile stationperforms handover from the source base station to the target basestation in a handover procedure, the source base station performswireless communication with the mobile station and transmits anidentifier of a core network node to which the source base station isconnected to the target base station in the handover procedure, and thetarget base station receives the identifier of the core network.
 49. Themobile communication system according to claim 48, wherein theidentifier of the core network is included in a handover requestmessage, and the source base station transmits the handover requestmessage to the target base station.
 50. The mobile communication systemaccording to claim 48, wherein the target base station sets a radioresource and transmits a handover response message to the source basestation in the handover procedure, and the source base station receivesthe handover response message from the target base station.
 51. Themobile communication system according to claim 48, wherein the sourcebase station transmits communication type information to the target basestation in the handover procedure.
 52. The mobile communication systemaccording to claim 51, wherein the communication type information is alabel indicating values of communication types.
 53. The mobilecommunication system according to claim 51, wherein the communicationtype information includes at least one of a guaranteed bit rate, amaximum bit rate, a delay, an error rate and a traffic class.
 54. Amethod for a communication system, the system comprising a source basestation, a target base station and a mobile station, the methodcomprising: performing, in the source base station, wirelesscommunication with the mobiles station that performs handover from thesource base station to the target base station in a handover procedure;and transmitting, in the source base station, an identifier of a corenetwork node to which the source base station is connected, to thetarget base station in the handover procedure.
 55. The method accordingto claim 54, wherein the identifier of the core network is included in ahandover request message, and the source base station transmits thehandover request message to the target base station in the transmitting.56. The method according to claim 54, further comprising transmittingcommunication type information to the target base station in thehandover procedure.
 57. The method according to claim 56, wherein thecommunication type information is a label indicating values ofcommunication types.
 58. The method according to claim 56, wherein thecommunication type includes at least one of a guaranteed bit rate, amaximum bit rate, a delay, an error rate and a traffic class.
 59. Asource base station in a mobile communication system in which aplurality of base stations and a plurality of core network nodes aredistributed in a plurality of areas, the source base station comprising:means for performing wireless communication with a mobile station thatperforms handover from the source base station to a target base stationin a handover procedure; and means for transmitting, to the target basestation, a first identifier of a core network node to which the sourcebase station is connected and a second identifier of an area to whichthe source base station belongs, in the handover procedure.
 60. A targetbase station in a mobile communication system in which a plurality ofbase stations and a plurality of core network nodes are distributed in aplurality of areas, the target base station comprising: means forperforming wireless communication with a mobile station that performshandover from a source base station to the target base station in ahandover procedure; means for receiving, from the source base station, afirst identifier of a core network node to which the source base stationis connected and a second identifier of an area to which the source basestation belongs, in the handover procedure.
 61. A mobile station in amobile communication system in which a plurality of base stations and aplurality of core network nodes are distributed in a plurality of areas,the mobile station comprising: means for performing handover from asource base station to a target base station in a handover procedure;means for performing wireless communication with the source base stationand the target base station; and means for transmitting, to the targetbase station, an identifier of a core network node to which a sourcebase station is connected and an identifier of an area to which thesource base station belongs, in the handover procedure.
 62. A targetbase station in a mobile communication system in which a plurality ofbase stations and a plurality of core network nodes are distributed in aplurality of areas, the target base station comprising: means forperforming wireless communication with a mobile station that performshandover from a source base station to the target base station in ahandover procedure; means for receiving, from a mobile station, a firstidentifier of a core network node to which the source base station isconnected and a second identifier of an area to which the source basestation belongs, in the handover procedure.
 63. A source base station ina mobile communication system comprising a target base station and amobile station, the source base station comprising: means for performingwireless communication with the mobile station that performs handoverfrom the source base station to the target base station in a handoverprocedure; and means for transmitting an identifier of a core networknode to which the source base station is connected to the target basestation in the handover procedure.